Water Fast Calculator - Fasting Duration & Weight Loss Calculator
Free water fast calculator & fasting duration calculator. Calculate water fasting metrics, estimated weight loss, ketosis status, and hydration needs. Our calculator uses metabolic calculations to provide accurate fasting analysis and personalized recommendations.
Last updated: October 19, 2025
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Fasting Duration
3 Days
(72 hours)
Estimated Weight Loss
2.7 kg
Estimated Fat Loss
0.6 kg
Hydration Recommendation
7.5 liters
Recommended total water intake during fast (3 liters/day average)
Calories Burned
4,610
Total
BMR
1618
cal/day
Metabolic Status
Likely in ketosis
Recommendations
- •Short-term fast: Stay well hydrated and monitor your energy levels
Water Fasting Facts:
- • Initial weight loss includes water weight (especially first week)
- • Ketosis typically begins after 12-24 hours of fasting
- • Metabolism may slow 10-15% after 3 days (normal adaptation)
- • Always consult a healthcare provider before extended fasts
Water Fast Calculator Features
Tracking
Days & Hours
Calculate exact fasting duration from start to end date
Calculations
Weight & Fat Loss
Estimates based on BMR, metabolism, and fasting duration
Metabolic State
Ketosis Timeline
Estimates when your body enters ketosis (typically 12-24 hours)
Water Intake
2-3 Liters/Day
Personalized hydration recommendations based on fast duration
Metabolism
BMR & Calories
Calculates basal metabolic rate and total calories burned
Guidance
Safety Tips
Personalized recommendations based on your fast duration and goals
Quick Example Result
For a 3-day water fast starting at 70 kg:
Fasting Duration
3 Days
Est. Weight Loss
~2.5 kg
Water Intake
7.5 L
How Our Water Fast Calculator Works
Our water fast calculator uses metabolic calculations and fasting science to estimate weight loss, ketosis status, and hydration needs. The calculation applies BMR formulas and fasting metabolism adjustments to provide accurate estimates for your water fasting journey.
Water Fasting Calculation Formulas
BMR (Mifflin-St Jeor): Men: 10×weight(kg) + 6.25×height(cm) - 5×age + 5BMR (Women): 10×weight(kg) + 6.25×height(cm) - 5×age - 161Daily Calories (Fasting): BMR × 0.95 (first 3 days) or × 0.85 (after 3 days)Fat Loss: (Daily Calories × Days) ÷ 7700 calories/kgThese formulas provide accurate estimates for water fasting metrics. Initial weight loss includes water weight (especially in the first week), while long-term loss is primarily fat. Metabolism adapts after 3 days, reducing by 10-15%.
Shows metabolic changes, ketosis onset, and weight loss patterns during water fasting
Understanding Water Fasting
Water fasting involves consuming only water (and sometimes electrolytes) for a period of time. During fasting, the body undergoes metabolic changes including glycogen depletion, ketosis onset, and metabolic adaptation. Understanding these processes helps plan safe and effective fasts.
- Initial weight loss includes significant water weight (especially first week)
- Ketosis typically begins 12-24 hours after starting a fast
- Metabolism may slow 10-15% after 3 days as the body adapts
- Proper hydration (2-3 liters/day) is essential during fasting
- Extended fasts (>7 days) should be medically supervised
- Breaking a fast properly is crucial to avoid digestive issues
Sources & References
- Journal of Clinical Investigation - Metabolic Effects of FastingResearch on metabolic changes and ketosis during fasting
- American Journal of Clinical Nutrition - Fasting and MetabolismStudies on metabolic rate changes during extended fasting
- National Institutes of Health - Fasting ResearchEvidence-based information on fasting and health
Need help with other health calculations? Check out our Breastfeeding Calorie Calculator and Body Recomposition Calculator.
Get Custom Calculator for Your PlatformWater Fast Calculator Examples
Fast Profile:
- Start Weight: 75 kg
- Fast Duration: 5 days
- Gender: Male
- Height: 180 cm
- Age: 35 years
Calculation Steps:
- Calculate BMR: 10×75 + 6.25×180 - 5×35 + 5 = 1,792 cal/day
- Fasting metabolism: 1,792 × 0.85 = 1,523 cal/day (after 3 days)
- Water weight loss: 75 × 0.03 = 2.25 kg
- Fat loss: (1,523 × 5) ÷ 7,700 = 0.99 kg
- Total estimated loss: 2.25 + 0.99 = 3.24 kg
Result: 5-day fast, estimated 3.24 kg weight loss (2.25 kg water, 0.99 kg fat)
Ketosis likely active, recommended water intake: 12.5 liters total (2.5 L/day)
24-Hour Fast Example
Short-term fast for beginners
Entering ketosis, minimal weight loss (~0.5 kg)
7-Day Fast Example
Extended fast with supervision
Full ketosis, significant weight loss (~4-6 kg)
Frequently Asked Questions
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